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Friday, August 9, 2013

The Determination of Calcium by Displacement Titration - Lu Le Laboratory


    A solution of the magnesium/EDTA complex is useful for the titration of cations that form more stable complexes than the magnesium complex but for which no indicator is available. Magnesium ions in the complex are displaced by a chemically equivalent quantity of analyte and the liberated magnesium ions are then titrated with Eriochrome Blake T as the indicator. Note that the concentration of the magnesium solution is not important; all that is necessary is that the molar ratio between Mg2+ and EDTA in the reagent be exactly unity.


Preparation of the Magnesium/EDTA Complex, 0.1M (sufficient for 90~100 titrations)

1.     To 3.72g of Na2H2Y2˙2H2O in 50 mL of distilled water, add an equivalent quantity(2.46 g) MgSO4˙7H2O.

2.     Add a few drops of phenolphthalein, followed by sufficient 0.1M NaOH to turn the solution faintly pink.

3.     Dilute to about 100mL with water. The addition of Eriochrome Blake T to a portion of this solution buffered to pH 10 should cause a dull violet color to develop.

4.     Moreover, a single drop of 0.01M Na2H2Y2 solution added to the violet solution should cause a color change to blue, and an equal quantity of 0.01M Mg2+ should cause a change to red.

5.     The composition of the original solution should be adjusted with additional Mg2+ or H2Y2- until these criteria are met.


1.     Weight a sample of the unknown( to the nearest 0.1mg) into a 500 mL beaker[1].

2.     Cover with a watch glass and carefully add 5 to 10 mL of 6M HCl. After the sample has dissolved, remove CO2 by adding about 50 mL of deionized water and neutralize with 6M NaOH until the red color is discharged.

3.     Quantitatively transfer the solution to a 500 mL volumetric flask, and dilute to the mark.

4.     Take 50.00 mL aliquots of the diluted solution for titration, treating each as follows: Add about 2 mL of pH 10 buffer,1 mLof Mg/EDTA solution, and 3 to 4 drops of Eriochrome Blake T or Calmagite indicator. Titrate[2] with standard 0.01M Na2H2Y to a color change from red to blue.


5.     Report the number of milligrams of CaO in the sample.


[1] The sample taken should contain 150 to 160 mg of Ca2+.

[2] Interferences with this titration are substantially the same as those encountered in the direct titration of Mg2+ and are eliminated in the same way.

Experimental Record

Concentration of Na2H2Y(aq)
Weight of MgSO4

=1.00x10-3 mol
Mole of MgSO4 was added
1x10-5 mol
Weight of CaO sample

=2.68x10-3 mol
Mole of CaO sample was added (assumed as 100%)
2.68x10-4 mol
Consumption of 0.010M EDTA solution
20.10 mL

21.11 mL

21.63 mL
21.28 mL
Theoretical Consumption of 0.010M EDTA solution
26.80 mL
CaO in sample
119.2 mg
CaO % in sample
79.46 %

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